1. Field of the Invention
The present invention is directed to a magnetic resonance imaging (MRI) tomography apparatus, also known as a nuclear magnetic resonance (NMR) tomography apparatus, operating according to the echo planar imaging method, wherein the chronological curve of the read-out gradient follows a cosine function. A method of operating such an apparatus is also disclosed.
2. Description of the Prior Art
Magnetic resonance imaging tomography devices are known for examining a subject, such as the human body, based on the phenomenon of nuclear magnetic resonance. These tomography devices include a main or static field magnet which aligns the nuclear spins in the human body, and a RF system for exciting the nuclear spins and for receiving the signals emitted as a result of the excited nuclear spins. Gradient coils are provided which proceed in the direction of the main magnetic field and which generate a magnetic field which changes linearly in this direction. These coils are used for slice selection and for the spatial allocation of the signals in the slice. Further gradient coils are also provided which respectively generate magnetic fields proceeding the direction of the fundamental field, but which change in two directions perpendicular thereto. By exciting these gradient coils, the phase of the signal induced in the RF system following the generation of the main field is influenced in the body region under examination, dependent on the distribution of the nuclear spins. It is thus possible to derive an image of a planar section of the body based on the distribution of the nuclear spins.
When gradient coils are used in a magnetic resonance imaging apparatus for fast pulse sequences, the portion of the higher audio frequencies contained in the spectrum of the pulse sequences increases from about 1 through 10 kHz. To avoid both increased power consumption and field distortions in this range due to the skin effect in solid conductors, stranded or cabled conductors can be used for the turns of the gradient coils. The individual saddles of a coil set for the gradient coils associated with a predetermined spatial coordinate must be connected to each other over optimally short paths and with low inductivity. The respective gradient coils allocated to the field gradients G.sub.x and G.sub.y, which are generally fashioned as saddle coils, therefore preferably each consist of a single cabled conductor. These gradient coils can be fashioned as saddle coils consisting of three sub-coils which are arranged in the same cylindrical surface a carrier member. The number of turns of the outer sub-coil relative to the middle sub-coil and relative to the inner sub-coil can increase in a predetermined relationship, such as approximately 1:1:4.
For producing tomograms in a magnetic resonance imaging apparatus according to the echo planar imaging (EPI) method, the image consists of picture elements in the section plane. The measured values are acquired in the Fourier plane, i.e., the Fourier transformation of the image content is measured. The image values in the locus plane are calculated from the measured data in the wave number plane by two Fourier transformations taken in perpendicular directions, such as the x-direction and the y-direction. The selection of the section plane of the body ensues by selective slice excitation with a section selection gradient, and simultaneous RF excitation. The entire data set for an image can be acquired with a single elementary pulse sequence. The phase coding gradient is constant in the read-out region, whereas the read-out gradient oscillates.
In this known method, the desired wave number is a sine function, whereas the read-out gradient proceeds according to a cosine function. The read-out gradient jumps to zero at its maximum amplitude at the start of the echo sequence which is defined by the phase coding gradient. This method is described in the article "Biological and Medical Imaging By NMR," Mansfield et al., Journal of Magnetic Resonance, Volume 29, pages 355-373 (1978), the pulse and field gradient timing diagram for two-dimensional echo planar imaging being shown in FIG. 9 of the article.
A magnetic resonance imaging tomography apparatus is known wherein one of the gradient coils forms a parallel resonant circuit in combination with a capacitor. Amplifiers are provided for charging the capacitor. A thyristor switch is provided for modulating the gradient field, the thyristor switch being connected between the gradient coil and the capacitor, and controlling oscillation in both directions. Echo signals are excited by 180.degree. RF pulses. The current in the resonant circuit is proportional to the gradient field. The current of the gradient coil, however, is higher than the current of the amplifier by the quality factor Q. This quality can be influenced by eddy currents and by temperature changes, and is thus not constant during operation. Moreover, the thyristor switch which controls the oscillation requires a relatively large outlay. Such an apparatus is described in U.S. Pat. No. 4,628,264.